Computers have dramatically decreased in size in the past decade, enabling the production of powerful mobile computers. The popularity of mobile computers is due largely to their portability. This obvious advantage is counterbalanced by the difficulty of providing a long-lasting and portable power source for mobile computers.
For many uses of mobile computers, it is necessary or desirable to provide a long-lasting supply of power. For example, in performing extensive fieldwork away from external sources of power, it is desirable to have a long-lasting, portable source of power. Electrochemical batteries are desirably used as a long-lasting and portable power source for mobile computers.
When using battery power, it is necessary or desirable to minimize total costs. The two main classes of batteries are primary (non-rechargeable) batteries and secondary (rechargeable) batteries. When selecting among types of primary batteries, one must consider the cost of the batteries and how long the batteries last, among other questions. When selecting among types of rechargeable batteries, one must also consider how many times the batteries can be recharged, how long it takes to recharge the batteries, how long a charge lasts, and memory effects of the batteries, among other questions.
Many mobile computers have addressed these concerns by having a rechargeable and long-lasting, but expensive power source. For example, the Hewlett Packard 620LX uses lithium ion batteries. Lithium ion batteries have a long shelf life, long operating life, high energy density, and good recharge characteristics. Moreover, lithium ion batteries can be fully recharged quickly, often within two hours.
This typical practice of using rechargeable lithium ion batteries in a mobile computer has several drawbacks. Lithium ion batteries are expensive and often must be purchased as product-specific components from the manufacturer of the mobile computer. Moreover, chargers for lithium ion batteries, for example "smart charger" chips, are complex because strict control of charge and discharge is required for both safety and long cycle life. The circuitry for these chargers is relatively expensive and bulky compared to other recharging technologies. Finally, capacity loss through self-discharge for lithium ion batteries is relatively high, about 8% per month.
Other electronic devices may use rechargeable nickel-cadmium or nickel metal hydride batteries. For example, the Philips 300 series provides an optional rechargeable nickel metal hydride battery pack. Nickel metal hydride batteries are rechargeable and offer a slightly cheaper alternative to lithium ion batteries. Other advantages of include the ability to deliver high current in short periods and the ability to deliver relatively constant levels of current.
The disadvantages of nickel cadmium batteries include pronounced memory effect and other cyclic loss of capacity, high capacity loss through self-discharge, relatively low energy density, and environmental problems related to the cadmium. Compared to nickel cadmium batteries, nickel metal hydride batteries have less memory effect and no environmental problems, but have worse capacity loss through self-discharge.
Some mobile computers, for example the Casio Cassiopeia E-10, use primary (non-rechargeable) batteries, including non-rechargeable alkaline manganese, lithium, nickel cadmium, and nickel metal hydride batteries. Alkaline manganese batteries, for example, have long operating life, long shelf life, and relatively high energy density. Moreover, primary alkaline batteries are widely available for purchase and come in standard sizes. Compared to other battery types, primary alkaline batteries are inexpensive.
The chief drawback to using primary batteries in mobile computers is that primary batteries are not safely rechargeable. To some extent, this drawback of primary alkaline batteries is offset by low replacement cost and ease of replacement. Nevertheless, mobile computers that consume power at a high discharge rate have frequently chosen rechargeable lithium ion, nickel cadmium, or nickel-metal hydride batteries over quickly consumed alkaline batteries.
Rechargeable alkaline batteries, such as RAM cells from Battery Technologies Inc., are increasingly used in mobile electronic devices. Rechargeable alkaline batteries offer the advantageous usage characteristics of primary alkaline batteries including high energy density and low cost, and also are rechargeable. Rechargeable alkaline batteries are desirably fully recharged, or "topped off," on a frequent basis. Conversely, deep discharge of rechargeable alkaline batteries is desirably avoided. Rechargeable alkaline batteries can be charged by constant voltage taper charging or by pulse charging. In constant voltage taper charging, a voltage regulator, maintaining constant voltage in a circuit to a rechargeable alkaline battery, supplies current that trickles off as the battery becomes charged and voltage in the battery increases. For example, a 3.3 constant voltage regulator may be used to taper charge a pair of 1.5 volt rechargeable alkaline batteries by supplying current that "trickles off" as the batteries become charged. For optimal charging of deeply discharged alkaline batteries, constant voltage taper charging requires 10 to 12 hours of charging, and is desirably done overnight. In pulse charging, a "smart" charger containing microchip controllers safely sends pulses of power to a rechargeable alkaline battery at voltages much higher than the voltages in taper charging. For this reason, pulse charging requires less time than constant voltage taper charging. Finally, compared to other rechargeable batteries, rechargeable alkaline batteries have a relatively low capacity loss through self discharge, about 0.5% per month.
Rechargeable batteries are charged using a battery charging mechanism including some combination of a power supply, charging circuitry, and batteries. Charging circuitry may be packaged with the batteries in a special battery pack, or included in a separate battery charging device. A battery charging mechanism is desirably implemented with small, inexpensive circuitry. Further, a battery charging mechanism is desirably implemented in a way that is consistent with the form of the mobile computer and the way the mobile computer is used, simplifying both production and use.
In mobile computers, existing battery charging mechanisms for lithium ion, nickel, and rechargeable alkaline batteries come in a variety of forms. In some mobile computers, charging circuitry is located within the mobile computer for charging batteries within the computer, either as part of a battery pack or as separate circuitry within the computer. In other mobile computers, the charging circuitry is located within an external battery charger for charging batteries outside of the computer.
Implementing an efficient battery charging mechanism within the existing use patterns of a mobile computer has been difficult, especially considering the challenge of fitting charging circuitry within increasingly small mobile computers. For example, the inconvenience of having to remove batteries discourages optimal charging of rechargeable batteries in an external battery charger. At the same time, however, the size of charging circuitry makes it non-optimal to locate it within small mobile computers for in-device charging. Limited space is desirably used for increased battery storage or other uses. Even if charging circuitry is placed within a mobile computer, attaching an AC adapter to the mobile computer for the sole purpose of charging constitutes an inconvenient step. Further, placing circuitry within a mobile computer for in-device charging hampers efforts to market accessories for mobile computers because the charging feature may not be wanted by all users.
To conserve limited battery life, mobile computers typically offer AC adapters that provide direct current to a mobile computer from an external power source. An AC adapter bypasses the battery system, conserving battery life. In addition, an AC adapter may be used to supply power to charge rechargeable batteries. An AC adapter may be plugged directly into the mobile computer or into a docking cradle for the mobile computer. The chief drawback of AC adapters is that their use is limited to areas with available external power sources. Although an AC adapter effectively conserves battery power, it hinders full utilization of the portability of mobile computers.
A docking cradle is a device that allows a mobile computer to communicate with a second computer, usually a desktop computer, when the mobile computer is connected to ("docked" at) the docking cradle. As is known in the art, a docking cradle is interchangeably referred to as a "docking station" or "cradle." In many mobile computers, an AC adapter plugged into the docking cradle powers the mobile computer while the mobile computer communicates with a second computer.